Custom Styles are also Style+ styles such as when any manual formatting of text is applied instead of properly using a style. This will lead to many additional Custom styles being created by most users.

Personally I try to encourage collaborators to only use the Outline View. Then when the document is passed to the editor, they can do the editing and layout before finalizing the document.
Beyond that, tell them to work in Notepad.

Design. Style Sets. Themes.